This invention relates to a tape cartridge, and more particularly, to an optical tape cartridge.
Much attention has been focused on storage on optical disks. This attention follows the invention of the modern optical disk by David Paul Gregg. As optical disk recording technology has matured, optical disk storage has become commonplace in the consumer market. For example, motion pictures, photographs, data, and music are stored on optical disks of suitable diameters. These various optical disks are distrubuted comercially under trade marks such as Laser Disc, Compact Disc, Mini Disc. These disks employ the same basic stucture and pulse modulation.
While optical disks are the predominant optical storage medium, another optical media, optical tape, has a greater storage capacity. Because of its greater storage capacity, optical tape is a more suitable medium than optical disks for certain storage applications, such as recording and playing programs for High Definition Television (HDTV).
One disadvantage of optical tape is its greater sensitivity to errors caused by dust, smoke or other minute particles, adhering to the tape and obscuring the reading beam. With an optical disk, the recording layer contains indicia having widths of less than 1.0 micrometers, recorded at a pitch of less than 2.0 micrometers. Dust particles are typically about 1 micrometer in diameter. Were dust particles in direct contact with the recording layer, the dust particles would likely obscure the underlying indicia. In optical disk technology, assuming the read or write laser beam has a numerical aperture of about 0.5, the diameter of the beam at the surface of the protective layer would be about 1 millimeter, much larger than the diameter of a typical dust particle.
Unlike an optical disk, an optical tape needs to be thin to be flexible and achieve higher storage density. The optical tape recording layer typically contains indicia that are as small or smaller than those of optical disks. Typical optical tapes are about 25 micrometers thick, including the thickness of the protective layer covering the recording layer. The optical tape protective layer is typically only about 2 to 10 micrometers thick, about 500 to 100 times thinner than that of the optical disk, and therefore much more vulnerable to dust obscuring the indicia in the recording layer.
One approach to dealing with dust and other particles is discussed in U.S. Pat. No. 5,272,689, issued to Tsujioka et al. This patent teaches an optical pickup that includes a transparent contact member that contacts the protective surface of the optical tape, sweeping dust particles away from the path of the beam. The contact member is in the optical path of the beam. Provision is made to adjust the focus of the objective lens to compensate for shortening of the optical path caused by wear of the contact member.
Another approach to the problem of dust obscuring the optical tape is suggested in U.S. Pat. No. 5,215,808, issued to Barnard. This patent teaches providing a small bubble of fluid (e.g., gas or liquid) trapped between the recording surface and the protective surface. During recording or playing, the trapped bubble displaces dust particles out of the focus zone of the read/write beam. As the tape moves, the trapped bubble is held in place under the read/write beam.
Another approach to the problem of dust and other minute particles is simply to house the optical tape in a sealed cartridge. U.S. Pat. No. 4,814,925 to Beaujean discloses a hermetically sealed cassette containing a recording tape. The cassette is filled with an inert low pressure gas which does not react chemically with the surface of the tape or with the surface of a tape guide means.
Sealing the cartridge, however, presents the difficulty of conveying mechanical motion to moving parts located within the sealed cartridge. One solution to this difficulty is taught by U.S. Pat. No. 5,077,724 to David Paul Gregg. This patent discloses a sealed optical tape cassette in which an optical tape is pulled across a mobile recording bridge situated behind a transparent window. The bridge is operated upon by a pair of sealed internal/external pins to control tape tilt, focus and tracking of a light beam from a spinning optical head in a recorder/player to a desired point on the optical tape.
There is therefore a need for an improved optical tape cartridge that is low cost, reliable, and suitable for storing HDTV recordings.